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Paris, 29 September 2011

The environment of a supermassive black hole revealed

An international team of astronomers, including two CNRS researchers(1), has revealed new data about the environment surrounding one of the brightest supermassive black holes known. The scientists have discovered a corona of very hot gas, with a temperature of around ten million degrees Celsius, swirling around the black hole. They have also revealed the existence of powerful winds made of cold, dense clouds of gas surrounded by hotter, more diffuse gas. According to the astronomers, these winds are blown away from the black hole and from the center of the galaxy at speeds exceeding 700 km/s. The results, which were achieved using five space telescopes, in particular ESA's XMM-Newton and INTEGRAL(2), are the subject of seven articles to be published in the journal Astronomy & Astrophysics from September 29. They should make it easier to interpret observations obtained for other supermassive black hole galaxies.

Supermassive black holes with masses several hundred million times that of the Sun are found at the center of most massive galaxies. Contrary to popular belief, they do not swallow up all the matter (gas and dust) that surrounds them. The gas and dust fall in towards the black hole, usually forming a disk that rotates around it. The infalling matter releases phenomenal amounts of radiation, mainly ultraviolet and X-rays. The emission of this radiation is sometimes so strong that it diverts part of this matter away from the black hole, forming winds that can reach several hundreds of km/s. However, the environment of supermassive black holes is still poorly known. What kind of matter surrounds them? Where and how are the nearby flows of matter formed?

An international team of astrophysicists has managed to observe, map and characterize, with hitherto unequalled precision, the environment of one of the brightest supermassive black holes known, located at the center of the distant galaxy Markarian 509. To obtain this unprecedented view of the central regions of Mkn 509, the researchers used five large space telescopes. The highlight of the campaign, carried out in 2009, was the repeated and simultaneous observation over a six week period of the radiation emitted by Mkn 509(3) right across the spectrum from visible to gamma-ray wavelengths, using ESA's XMM-Newton and INTEGRAL spacecraft.

Hot corona as energy converterFirst result: this supermassive black hole, whose mass is 300 million times that of the Sun, is surrounded by a disk of gas that emits ultraviolet radiation(4). The researchers observed a very hot gas (with a temperature of several million degrees Celsius) forming a corona hovering above the disk. The corona absorbs the ultraviolet radiation and re-emits it at higher energies, in the region of low-energy X-rays (radiation that is nonetheless several hundreds of times more energetic than visible light). The discovery of the existence of this very hot corona enables researchers to better understand observations performed for other active galaxies (galaxies that harbor supermassive black holes in their center), which were previously difficult to decipher.

Cold, dense gas cloudsFor the first time, the scientists have shown that the outflow of matter blown away from the heart of Mkn 509 is made up of at least five distinct components, with temperatures ranging between 20 000 and 1 million degrees Celsius. They have also found that most of the gas present in these winds comes from regions located around 15 light years from the central black hole. The winds are made up of cold, dense clouds of gas surrounded by hotter, more diffuse gas.

Signs of collision between galaxiesIn addition, the researchers collected data on the interstellar gas of the host galaxy, Mkn 509. This gas is strongly ionized by the emission of X-rays coming from the central X-ray source: the atoms are stripped of most or all their electrons when irradiated by a powerful X-ray source. This has revealed the presence of the gas several hundred thousand light years from the central black hole. Falling towards Mkn 509 at a velocity of several hundred km/s, this gas may result from a past collision with a smaller galaxy, a collision that may have caused Mkn 509's current activity.

Gas and dust swirling around a black hole. Although part of this matter is attracted by the black hole and will eventually be swallowed up, another part is blown away, forming winds.

Notes:

1 - In France, the two laboratories involved are the Institut de Planétologie et Astrophysique de Grenoble (CNRS/Université Joseph Fourier) and the Institut de Recherche en Astrophysique et Planétologie (CNRS/Université Paul Sabatier). These works were partly funded by CNES. The main INTEGRAL instruments (SPI and ISGRI) used in this campaign were developed by CNES and CEA, which are also involved in their in-flight calibration and monitoring.
2 - European Space Agency.
3 - The LETG spectrometer on NASA's Chandra X-ray satellite and the COS spectrometer on the Hubble Space Telescope were used to complete these observations. The Swift X-ray satellite was also used prior to and following these observations in order to monitor the behavior of the source before and after the campaign.
4 - Radiation that is a little more energetic than visible light, but far less energetic than X-rays.